NIL                      Predefined Value for an Undirected Pointer

 <pvar> := NIL;   { assignment }
 <pvar>  = NIL    { comparison: equality }
 <pvar> <> NIL    { comparison: inequality }

    NIL is the standard value for any pointer variable that does not
    currently point to a valid data structure. You should set all such
    pointers to NIL, then test for that value before referencing the
    pointer.

  -------------------------------- Example ---------------------------------

           { look through linked list for last entry }
           P := Head;
           while P^.Next <> NIL do
             P := P^.Next;

           Tail := NIL;